DACs are used to convert a digitally coded signal to an analog signal, or in conjunction with successive approximation circuitry as part of an analog-to-digital converter. DACs convert a digitally coded signal to an analog signal, typically a voltage, that corresponds to the digitally coded signal. The analog signal can take on many different values over a predetermined range that corresponds to the range of digitally coded signals.
DACs may employ a resistor string comprised of series coupled equal resistance resistors. Between contiguous resistors in the resistor string, as well as between the resistor string and an energy source energizing the resistor string, are intermediate taps. Switches, coupled between an output node and intermediate taps, when turned on electrically couple the respective intermediate taps to the output node, and isolate the intermediate taps from the output node when turned off.
The precision with which resistors are formed affects the precision of the resulting analog signal. A shortcoming of prior art DACs has been the difficulty of fabricating a resistor string in which each resistor has the same resistance, particularly in applications where the resistor string layout includes one or more direction reversals. Direction reversals are introduced to constrain a resistor string to an area on an integrated circuit that results in a chip with an acceptable aspect ratio. Inaccuracy in fabricating the resistors results in incorrect analog signals being produced at the output of a DAC.